It is important that future wireless and/or cellular systems can offer, apart from many other aspects, increased coverage, higher data rates or a combination of both. In addition, the cost aspect of building and maintaining the system is expected to become even more important in the future. As data rates and/or communication distances are increased, the problem of increased battery consumption also needs to be addressed.
An important aspect is rethinking the topology used in existing systems, as there has been little change of topology over the three generations of cellular networks. In this respect, the introduction of so-called relaying networks, such as multi-hop networks and two-hop relaying networks, has been a great leap in the right direction.
For instance, it is well known that so called multi-hopping offers possibilities of significantly reduced path loss between communicating (relay) entities, which may benefit the user. When routing is applied in a wireless communication network, such a network is often denoted a multi-hop network. In a multi-hop network, nodes or stations out of reach from each other can benefit from intermediately located nodes that can forward their messages from the source node towards the destination node. Traditionally, multi-hop networks have been associated with so called ad hoc networks, where nodes are mostly mobile and no central coordinating infrastructure exists. However, the idea of multi-hop networking can also be applied when nodes are fixed and/or a central coordinating infrastructure exists. One such scenario targets rural area Internet access and uses fixed nodes attached to the top of house roofs, lamp posts and so forth.
In a multi-hop scenario, information may be transmitted over multiple hops between source and destination rather than directly in a single hop. In general, the multi-hop approach offers several advantages such as lower power consumption and higher information throughput compared to a direct one-hop approach. In a multi-hop network, nodes out of reach from each other can benefit from intermediately located nodes that can forward their messages from the source towards the destination.
A related approach to provide enhanced coverage and data rate is to use so-called two-hop relaying, which could be viewed as a degenerate case of multi-hopping involving only two hops, but at the same time generalized to and allowing for parallel paths if desired. Many different variants of two-hop relaying exist.
In a specific form of two-hop relaying, a transmitter sends a signal to a relay, which receives the message and then forwards it to a receiver, either by regenerative relaying (decode-and-forward) or non-regenerative relaying (amplify-and-forward). A major benefit of two-hop relaying stems from splitting a long transmission distance into two roughly equidistant hops to allow increased data rate on each link as well as increased end-to-end (ETE) total data rate.
In another form of two-hop relaying, a transmitter sends a signal to a receiver, but also to a relay, which receives and forwards the message to the receiver. The receiver then combines the first direct signal and the second relayed signal to enhance the quality of the signal, which means increased average data rate (and less variance due to diversity). This form of relaying is sometimes referred to as cooperative relaying.
A more elaborate form of cooperative relaying, however, employs various aspects of “cooperation” among several relay nodes. For example, a signal sent by a transmitting node may first be received by multiple relays, and subsequently and concurrently forwarded, and finally received by a receiving node.
In cooperative relaying, the relays are generally allowed to perform various signal processing or coding tasks that in different ways improve the overall communication performance. The benefits of the mechanisms that are exploited in cooperative relaying can broadly be divided into diversity gain, beam-forming gain, and spatial multiplexing gain. Also here, the receiver could enhance the quality of the signal by combining the direct signal and the relayed signals.
In recent research literature, cooperative relaying goes under several names, such as cooperative diversity, cooperative coding, virtual antenna arrays, and so forth. A good general overview of cooperative communication schemes is given in reference [1]. The general benefits of cooperation between stations or nodes in wireless communication can be summarized as higher data rates, reduced outage and variance (due to various forms of diversity), increased battery lifetime and extended coverage.
The use of traditional repeaters can also be considered as two-hop relaying in its simplest form. A repeater is often a relatively simple relay node, which only offers fairly rudimentary functions such as amplify-and-forwarding and perhaps power control. The boundary between repeaters and more advanced relays is, however, not sharp. The terms “relay” and “repeater” are often (and will also here be) used interchangeably.
An example of a traditional two-hop relay system is shown in FIG. 1. The two-hop relay system or network basically comprises a transmitter (TX) 10, a receiver (RX) 30, and one or more relay stations (RS) 20. When the number V of relays is greater than one, or alternatively, when both a direct signal and a relayed signal are exploited, this represents the cooperative relaying case.
There is a general demand for improved performance of relaying networks and the involved relay components.